removing an accidentally merged branch

[Harvey Chapman]

I accidentally ended up with the following graph. How do I permanently get rid of rev 689? It must be removed since this is used with hgsubversion and I can't push it. Is there a way to remove the merge at 691 so that I could then just strip 689? I've tried using mq and also bundles, but I must be doing something wrong. Thank you.

o  changeset:   692:99482322f98b
|               Adding /dev/hd* and /dev/sd* mdev debugging.
o    changeset:   691:4010971da1bf
|\                Use a sequence number with mdev so partition table reloading will
| |  		work correctly.
| o  changeset:   690:66b7f25a8807
| |               Logging hasn't been optional for a while. Turn off the check which
| |  		could have become a bug.
o |  changeset:   689:6da837a7ff67
|/                Logging hasn't been optional for a while. Turn off the check which
|    		could have become a bug.
o  changeset:   688:df2de96239ce
|               Add .DS_Store mac file to ignore